This achievement will take a lot of time and a bit of luck to get. There are two basic ways to go about it: breeding or inviting. For breeding you need to find at least one, and preferrably two, dwarfs to marry or invite into your court. There is a 15% chance that any child of a dwarf will also be a dwarf so marry your dwarfs to other dwarfs to increase your chances and eventually you'll get there.
The other way is to invite and marry as many dwarfs as possible into your court. If your empire gets large enough this becomes progressively easier, especially if you can take multiple wives/concubines since your vassals are more likely to accept your request than independent rulers. Taking concubines is the easiest way here if the pagans are powerful enough since you can immediately set them aside and they will remain in your court and your can take concubins from any other pagan religion.
Obviously a hybrid of the two ways will be the best. I played as Norse and waited until the 1100s when I had conquered most of europe and then took a bunch of dwarf concubines and married them to any male dwarfs I could get into my court. After a couple decades I had enough and the achievement unlocked.